The recent explosion of small and portable wireless battery-powered devices (hereinafter, “WBPDs”), such as cellular phones and personal digital assistants (PDAs), has escalated both the desire to exchange information directly between such devices and the desire to obtain access to conventional network and application services from these devices. Due to the relatively short battery life of these devices, tradeoffs must be made between the available network bandwidth and the wireless transmission range. The need to support reasonable bandwidth for interactive applications, multimedia, and so forth on these devices consequently drove the emergence of short-range communication technologies.
Examples of these short-range communication technologies include IEEE 802.11, HomeRF, and Sharewave. (IEEE 802.11 is a standard of the Institute for Electrical and Electronics Engineers, which was approved in 1997 for wireless Local Area Network, or LAN, signaling and protocols. HomeRF and Sharewave are directed towards in-home networking solutions. More information on these techniques can be found on the Internet at www.ieee.org, www.homerf.com, and www.sharewave.com, respectively.)
Devices using these prior art technologies typically comply to a wireless peer-to-peer networking protocol, allowing for multiple simultaneous device interactions of ranges of 50 meters or more, with data rates well above 1 Megabit per second (Mbps). However, the power consumption requirements for these technologies are too high to make them good candidates for use with WBPDs. Consequently, simpler, less power demanding wireless networking technologies are beginning to appear. Unfortunately, to achieve the reduced power requirements, these newer technologies place more severe restrictions on the distance and data rate of the wireless communication, as well as on the number of devices that may communicate simultaneously.
One of the most recent examples of these newer wireless technologies is known as “Bluetooth”. (Refer to location www.bluetooth.com on the Internet for more information on Bluetooth.) Bluetooth was initially designed to replace cables between closely located devices. Bluetooth technology is therefore optimized for short-haul, point-to-point connectivity. Bluetooth-enabled devices also consume less power than devices that are designed according to the older wireless technologies such as 802.11. Because of these factors, Bluetooth is one obvious candidate for use with WBPDs.
However, although Bluetooth-enabled devices have a number of beneficial attributes, the Bluetooth design point leads to severe disadvantages when compared with the older wireless technologies in terms of the previously-stated factors of distance, data rate, and number of communicating devices.
The disadvantage in terms of distance can be seen by comparing Bluetooth to 802.11. 802.11 technologies provide connections between a base station and end devices over distances that may exceed 100 meters. Bluetooth connects across much shorter distances, typically less than 10 meters for those devices classified in the Bluetooth standard as “Class 3 devices.” (Class 1 Bluetooth devices are the most powerful devices defined according to the Bluetooth standard, and they provide connections over distances up to 100 meters; however, these Class 1 devices require considerably more power than Class 3 devices.) The working distance between Class 3 and Class 1 Bluetooth devices is required to be 10 meters or less. (This is because different class devices must tune their receivers to have equal sensitivity; in this case, they must tune to the sensitivity of the Class 3 device, on the order of −70 decibels relative to 1 milliwatt, or −70 dBm.)
In terms of data rate, the older wireless technologies provide much higher data rates than Bluetooth. 802.11, for example, offers data rates to up to 11 Mbps. Bluetooth, on the other hand, has a shareable raw data rate of less than 1 Mbps. For pure asynchronous connection-less (ACL) links, the effective data rate is approximately 720 Kilobits per second (Kbps). A device may participate in up to three synchronous connection-oriented (SCO) links, in which case the effective ACL data rate is reduced to less than 440 Kbps.
Regarding the third disadvantage, the number of devices supported in the older wireless technologies is typically limited only by the performance of the individual end devices that are communicating via the shared wireless LAN. The number of Bluetooth devices, however, is severely restricted by the Bluetooth architecture. The architecture defines master and slave roles for a group of devices connected in an ad-hoc network configuration referred to as a “piconet,” and it specifies that a device serving as a master in a piconet may not control more than 7 active slave devices at one time.
In spite of the limitations of existing wireless connectivity solutions suitable for personal WBPDs, users have an ever-increasing desire to connect their personal WBPDs to networks, including large Internet Protocol (IP) networks, home networks, corporate LANs, and the Internet.
Two approaches have emerged for satisfying these consumer demands. In the first solution, an intermediary device that is customarily referred to as an “Access Point” (AP) has been produced by several companies. In the second solution, a “scatternet” approach is used to expand a network to accommodate multiple WBPDs. Each of these approaches will now be described.
FIG. 1 illustrates a networking environment according to the first approach, in which several access points 140, 141, . . . are used to connect end user devices such as those depicted at 130, 131, and 132 to an IP data network 120. (The IP data network in this prior art scenario may be either wired or wireless; the distinction is not pertinent to the present discussion.) These AP devices of the prior art may be based on a conventional computing device, which has both Bluetooth and LAN network adapters (for communicating with end devices 130, 131, 132 and network 120, respectively). Or, the AP device may be a special-purpose, dedicated device that combines both of these adapters and provides the necessary bridging functions between them.
However, these existing Bluetooth-based access points have distinct disadvantages when compared to longer-range wireless communication solutions, such as 802.11, or wire area wireless communications. While an AP based on Bluetooth technology (and its 10 meter limitations) may be fine for home or small office environments, these disadvantages make it a less attractive choice as compared to a wireless LAN for serving a crowded wireless environment like a conference room (or for serving a large space such as an airport or school) as will now be described.
A first disadvantage of Bluetooth AP solutions is that the number of APs may need to be excessively large due to limitations of the Bluetooth design specifications. A single AP (i.e. a Bluetooth Class 3 device) operation in the role of a master within a piconet can serve an areas having a 10 meter radius, providing a circular area of coverage of approximately 314 square meters. Adapting this circular coverage area to a conference room having a square shape (while covering the entire room), one AP can serve a square room of approximately 14.2 meters by 14.2 meters, or 200 square meters. In compliance with the Bluetooth specifications, however, this AP can support only 7 active Bluetooth slave devices. Hence, if the conference room has 50 seats and all the users seated in the room must be provided with on-line connections at the same time, then the minimal number of required APs is 8. (In the general case, the number of APs required to support N users distributed within the circular coverage range of a single AP is the ceiling of the expression N/7, or ┌N/7┐.
Second, the number of APs may depend on the shape of the place in which the APs are used. For example, if the same 200-square meter conference room in the above example has an elongated or L-shape, rather than a square shape, then coverage with 8 APs is no longer sufficient. Suppose that the dimensions of the room are 10 meters by 20 meters. Because the lengthwise distance of the room is longer than 14.2 meters, a single AP is no longer capable of serving users located throughout the room. To provide network connectivity throughout this rectangular shaped 200-square meter room, the number of APs must be doubled to 16 because it may happen that all 50 users are crowded first at one end of the room and later at the other end.
Third, the number of APs may need to be even larger, because the locations of the end devices may be dynamic and unpredictable, and network capacity must be able to accommodate the worst-case number of active users as the users move around the room. For example, suppose APs are used within a museum to provide information to museum visitors as they view each painting within a large open space. To ensure that each museum visitor can make a connection as he views each painting, enough network capacity must be provided at each painting to support the entire group of visitors, even though most of the time little to none of this capacity is required (i.e. most of the time, no users or a very small number of users is standing in front of any one painting).
Another, and perhaps the most important, disadvantage of Bluetooth AP solutions is that the installation of APs requires additional wiring, making the infrastructure installation very expensive. Each AP requires an additional power plug and an additional network connection or port. In other words, a Bluetooth network using APs may require a large number of wired jacks, making the supposedly “wireless” network wireless only for the end devices. Extensive wiring causes high labor costs, and may possibly even surpass the cost of the APs in the near future, as AP cost decreases with proliferation of the technology. As a result, installation of a Bluetooth AP network may be as costly as installation of a fully wired network.
A further disadvantage of building a network infrastructure using Bluetooth APs is that such networks are not easily scalable to add more users, nor are they easily reconfigurable to modify the area of coverage. Both types of changes will typically require installation of additional wiring to support additional APs or movement of existing APs to new locations (in which case, new wiring may be required at the new location).
Using a wireless LAN such as 802.11 in the AP configuration, instead of Bluetooth, does not eliminate these problems: Power wiring is still required. Furthermore, in an area where Bluetooth communication is already in use, it may not be a good idea to introduce a wireless LAN using another technology because the wireless LAN shares the same radio spectrum (i.e. 2.4 GHz, the ISM) as Bluetooth, along with microwaves, cordless telephones, and other technologies. Interference in this environment may lead to unacceptably low performance of both networks when a large number of wireless dongles is concentrated in a space that provides service to a large number of users.
The scatternet approach of the second solution to providing network access for multiple WBPDs uses multiple piconets communicating with each other in a daisy-chained fashion, wherein an end device may play a dual role of a master in one piconet and a slave in another. One AP may support more than 7 end devices in this configuration. For example, if a first piconet lacks an AP and therefore cannot communicate with network, but the master of this first piconet is within range of a second piconet which does have an AP connected to the network, then the devices of the first piconet can communicate with the network (and with the devices of the second piconet) by virtue of the first piconet's master also playing the role of a slave in the second piconet. Additional wiring is not required in this scatternet approach, because any of the end devices may serve as bridges between the two piconets and thereby extend the reach of the network.
On a negative side, however, such an easy solution to scalability may cause a substantial diminishing of performance. Continuing the previous example of 50 users in a room, if this room is served by a single AP that is daisy-chained to multiple piconets, the daisy-chaining approach leads to a degradation of the useful payload rate to less than {fraction (1/50)}th of 720 Kbps, or about 10-14 Kbps. This number is comparable with wide-area network data rates, which do not require any additional hardware to install. Moreover, network delay is longer for those devices that connect to the network through the daisy chain. For example, if the connection set-up time within one piconet is from 1 to 5 seconds, then the last user in a daisy-chained scatternet comprised of 8 piconets will require approximately 8 to 40 seconds before being connected to the network.
Furthermore, if one or several of the portable end devices serving as masters in the ad-hoc daisy chain fails, is turned off, or leaves the room, then the rest of the chain will be cut off from the network permanently or temporarily (i.e. until one of the slave devices is found that can become a master). When this happens, considerable complexity is introduced to maintain an up-to-date routing structure for data through the scatternet between the APs and the end devices.
Using a scatternet also leads to unpredictable data rates and latencies for the end devices, because the number of daisy-chained piconets is unpredictable. There is no minimum guaranteed performance for the end devices positioned lower in the chain, because it will depend on the number of devices that are higher in the chain and the resources they consume. Moreover, as the piconet routing structure changes, as described above, the data rates and latencies may also change dynamically.
Accordingly, what is needed is an efficient, cost-effective technique for enabling multiple WBPDs to connect to a network that avoids the limitations of prior art techniques.